Biomedical subjects

S H Chan

Publications and source records attributed to S H Chan.

At least 109 records · Page 6Linked to original sources

PubMed1996

Tumour suppressor p53 and Rb genes in human hepatocellular carcinoma.

Aberrations of the p53 and Rb tumour suppressor genes were examined in 12 human hepatocellular carcinoma (HCC)-derived cell lines from different geographic areas and 9 local HCCs by restriction fragment length polymorphisms (RFLP), polymerase chain reaction-single-strand conformation polymorphisms (PCR-SSCP) and DNA sequencing. The relationships between genetic changes and hepatitis B virus (HBV) DNA integration in samples were compared. None of the cell lines and tumours showed structural changes in the Rb gene, while 6 cell lines and 2 tumours had mutation or deletion in exons 5 to 8 of p53. Mutations include an AGG --> AGT (Arg --> Ser) transversion at codon 249 in PLC/PRF/5 and Mahlavu, an AAT --> AAA (Asn --> Cys) transversion at codon 200 in TONG/HCC, an AAG --> GAG (Lys --> Glu) transition at codon 139 in HCC-T, a CAT --> CGT (His --> Arg) transition at codon 214 in SC4, and a CCC --> CTC (Pro --> Leu) transition at codon 250 in SC8. In Huh4, an 18-bp deletion from codon 264 to 270 resulted in loss of Leu-Gly-Arg-Asn-Ser-Phe from the amino acid sequences 265 to 270, whereas Hep3B had a 7-kb deletion after exon 7 of p53. Our data indicate that whereas Rb may not have pleiotropic effects on HCC, p53 aberrations are frequently involved in hepatocarcinogenesis. Further, HBV infection appears to be unrelated to the micro-genetic changes of p53. The G to T codon-249-mutation is consistent with HCCs arising from areas at high risk for both aflatoxin B1 (AFB1) exposure and HBV infection.

Animals↗

PubMed1996

C4 and 21-hydroxylase gene deletions in nasopharyngeal carcinoma among the Chinese.

Deletions around the C4 and 21-hydroxylase (21-OH) gene region occur among the Chinese, and C4B together with 21-OHA deletion is the most common. This deletion was found to be strongly associated with human leucocyte antigen (HLA) B58 DRB1*0301, particularly with DRB1*0301. The initial observation of a high frequency of this deletion in nasopharyngeal carcinoma (NPC) was secondary to the B58 DR3 association with NPC. A deletion of 21-OHA alone was observed to be associated with HLA B13 DRB1*1501.

Asian People↗

PubMed1996

Biomarkers: a molecular approach to cancer epidemiology.

The rapidly evolving field of molecular epidemiology represents a paradigm shift in medical research, illustrating the convergence of basic, clinical and epidemiologic research, as well as the application of laboratory methods to population studies. As a complement and improvement to traditional epidemiologic approaches, the use of biomarkers offers more accurate exposure assessment at an individual level, better understanding of the biological mechanism of carcinogenesis by tracking a continuum of events between exposure and disease, as well as the ability to discern host or genetic factors that may account for variations in cancer susceptibility among individuals or ethnic populations. Further, biomarkers have also proved useful in cancer diagnosis and prognosis. This article attempts to review the latest progress in molecular cancer epidemiology and to summarise the current state of research in Singapore.

Biomarkers, Tumor↗

PubMed1996

Molecular diagnosis of nasopharyngeal carcinoma: a review.

The various antigen complexes of the Epstein-Barr virus (EBV) are broadly classified as the viral capsid antigen (VCA), diffuse early antigen (EA-D), restricted early antigen (EA-R), membrane antigen (MA) and the Epstein-Barr nuclear antigen (EBNA). The different EBV-related diseases may be differentiated according to the reactivity of these different classes of antibodies towards the various classes of antigen complexes. However, with the recent development of molecular biology, it is now known that the individual polypeptides of the different EBV antigen complexes can be used as serological markers for the detection of nasopharyngeal carcinoma (NPC). Among the useful serological markers which have been used in enzyme-linked immunosorbent assay (ELISA) for the detection of NPC are the gp125 from the VCA complex (IgA), pp58 from the EA-D complex (IgG), ribonucleotide reductase (IgG and IgA), DNase (IgA) and thymidine kinase (IgA) from the EA-R complex, gp 250/200 from the MA complex (IgA) and the ZEBRA antigen (IgA).

Antigens, Viral↗

PubMed1996

Participation of AT1 and AT2 receptors in the differential interaction between angiotensin II or III and alpha-2 adrenoceptors in the nucleus reticularis gigantocellularis in cardiovascular regulation and antinociception in rats.

We evaluated possible interactions between angiotensin II (AII) or angiotensin III (AIII) and the alpha-2 adrenoceptors of the nucleus reticularis gigantocellularis (NRGC) in the medulla oblongata that are involved in cardiovascular regulation and antinociception, as well as the angiotensin receptor subtypes involved, using Sprague-Dawley rats that were anesthetized with pentobarbital sodium. The efficacy of guanabenz, which acts on the alpha-2 adrenoceptors in the NRGC to elicit hypotension, bradycardia and antinociception, based on tail-flick responses to noxious thermal stimuli (50 degrees C), was used as our experimental index. Bilateral microinjection of AII or AIII into the NRGC, at equimolar doses (40 pmol) that did not alter base-line systemic arterial pressure, heart rate or tail-flick latency, significantly and site-specifically attenuated the cardiovascular suppression elicited by guanabenz (100 micrograms/kg i.v.). This attenuation was appreciably antagonized by coadministration of the AT2 receptor antagonist PD-123319 (1.6 nmol). Concomitant examination of tail-flick responses revealed discernible inhibition by AII (40 pmol), but potentiation by AIII (40 pmol), of guanabenz-induced antinociception. These differential modulating effects of AII and AIII were, however, antagonized by comicroinjection of losartan (1.6 nmol) into the bilateral NRGC. Our results suggest that both AII and AIII produced a reduction, via AT2 receptors, of the activity of alpha-2 adrenoceptors in the NRGC that are involved in central cardiovascular regulation. On the other hand, antinociception induced by activation of alpha-2 adrenoceptors in the NRGC was suppressed by AII and potentiated by AIII, although AT1 receptors may play a major role in both interactions.

Analgesia↗

PubMed1995

Inhibition by intrathecal prazosin but not yohimbine of fentanyl-induced muscular rigidity in the rat.

We evaluated the effect of intrathecally administered prazosin, alpha 1-adrenoceptor antagonist, or yohimbine, alpha 2-adrenoceptor antagonist, on fentanyl-induced muscular rigidity. Adult, male Sprague-Dawley rats were anesthetized with ketamine and were under mechanical ventilation. Fentanyl given intravenously (100 micrograms/kg) or microinjected into the bilateral locus coeruleus (LC) (2.5 microgram/50 nl) consistently evoked a significant increase in the electromyographic activity recorded from the sacrococcygeus dorsalis lateralis muscle. This implied muscular rigidity was appreciably antagonized by prior intrathecal (10 microliters) administration of prazosin (5 or 10 nmol), but not equimolar dose of yohimbine. These results suggest that the spinal alpha 1-adrenoceptors in the coerulospinal noradrenergic pathway play a key role in fentanyl-induced muscular rigidity.

Adrenergic alpha-Antagonists↗

PubMed1995

Involvement of potassium and calcium channels at the locus coeruleus in fentanyl-induced muscular rigidity in the rat.

Previous work from our laboratory suggested that Go alpha protein at the locus coeruleus (LC) may be involved in the signal transduction process that underlies muscular rigidity induced by fentanyl. The present study further evaluated the roles of K+ and L-type Ca2+ channels, gating of which is known to be associated with activation of Go alpha protein, in this process, using Sprague-Dawley rats anesthetized with ketamine. Bilateral microinjection into the LC of tetraethylammonium chloride (100 or 200 pmol), a K+ channel blocker, and S(-)-Bay K 8644 (0.5 nmol), a Ca2+ channel activator, produced significant antagonization of the EMG activation elicited by fentanyl (100 micrograms/kg, i.v.), as recorded from the sacrococcygeus dorsalis lateralis muscle. On the other hand, local application to the bilateral LC of diazoxide (10 or 20 nmol), an ATP-dependent K+ channel activator, and nifedipine (0.25 or 0.5 pmol), a L-type Ca2+ channel blocker, was ineffective in blunting fentanyl-induced muscular rigidity. These results suggest that activation of K+ channels and/or inhibition of L-type Ca2+ channels secondary to triggering of the Go alpha protein at the LC may underlie the signal transduction process in the mediation of fentanyl-induced muscular rigidity.

Animals↗

PubMed1995

HLA-A2 allelic microvariants in nasopharyngeal carcinoma.

HLA-A2-positive nasopharyngeal carcinoma (NPC; n = 38) and normal control (n = 51) populations were analysed by high-resolution oligotyping to identify A2 allelic microvariants. Within the control group, A*0207 was found to be the most common allele, accounting for 50% of the A2 frequency in normals. In contrast, this allele was found to be present in only 23% of NPC cases, suggesting a protective effect. Of these 9 NPC patients with A*0207, all were associated with B46, unlike in the control group, where it can be found associated with non-B46 antigens. Another allele, A*0201, which was thought to be protective against NPC was in fact present in 39.5% of NPC patients, more than twice the frequency in controls. These data confirm that A*0201 is not a protective allele for NPC, and other factors such as the A*0207, non-B46 haplotype are of greater importance.

Alleles↗

PubMed1995

Isolation of the rat F1-ATPase inhibitor gene and its pseudogenes.

Multiple mitochondrial ATPase inhibitor genes have been identified in the rat-genome. The sequences of two genomic clones indicate that one encodes the functional gene, and the other is a processed pseudogene. The ATPase inhibitor gene isolated is about 1.5 kb long and the coding region contains three exons and two introns. The presence of multiple pseudogenes in the rat is suggested by this study and this is unique since in the bovine genome only a single gene has been found, which is also confirmed here. The presence of multiple inhibitor transcripts in the rat suggests that the functional gene might have multiple transcriptional start sites.

Amino Acid Sequence↗

PubMed1995

Cloning and characterization of two processed p53 pseudogenes from the rat genome.

We have cloned, by the polymerase chain reaction (PCR), two rat genomic fragments of 1.3 and 1.2 kb, both of which hybridize to a human p53 cDNA probe. Nucleotide sequencing revealed that they are two intronless rat p53 pseudogenes (designated as psi R53-1 and psi R53-2, respectively), representing a start-to-stop-codon-length copy of the processed transcript of the rat p53 gene. Further PCR analysis of DNA from rat organs (skin, kidney, etc.) of two different strains demonstrated that psi R53-1 and psi R53-2 were formed in the germ-line. As psi R53-1 and psi R53-2 share 85 and 83% homology with the rat p53 cDNA, it is thus estimated that psi R53-1 was created approx. 10 million years (Myr) ago and psi R53-2 arose 12 Myr ago. Moreover, GenBank scanning indicated that a 95-bp insert in psi R53-1, as compared with the cDNA, was 90% homologous with a sequence of mouse alternatively spliced p53 mRNA, where the spliced p53 mRNA contains an additional 96 nt derived from intron 10. Although the rat alternatively spliced p53 mRNA has so far not been described, our data suggest that these two processed pseudogenes may have been generated by integration of different mRNA intermediates into germ-line DNA.

Animals↗

PubMed1995

Concentration and regional distribution of propofol in brain and spinal cord during propofol anesthesia in the rat.

We evaluated the pharmacokinetics and regional distribution of propofol in the brain and spinal cord during propofol anesthesia in Sprague-Dawley rats, using high-performance liquid chromatographic determination of propofol concentration in brain, whole blood and plasma. We found that the concentration of propofol in the brain increased and decreased expeditiously during and after a 15-min and 30-min period of i.v. infusion of an anesthetic dose (60 mg/kg per h) of propofol. Furthermore, propofol was evenly distributed in the brain and spinal cord during infusion, with a significant inter-individual variation. Upon the establishment of anesthesia 15 and 30 min following intravenous infusion of propofol, the concentration of propofol in the brain, whole blood and plasma was respectively 15.7 +/- 1.9 and 39.4 +/- 2.7 micrograms/g, 4.5 +/- 1.2 and 13.6 +/- 1.3 micrograms/ml and 1.8 +/- 0.5 and 5.1 +/- 0.9 micrograms/ml (mean +/- SEM, n = 6 or 7). These high brain/blood and brain/plasma ratios during anesthesia suggest that propofol manifests a pharmacokinetic profile that is different from at least thiopental.

Anesthesia↗

PubMed1995

Adenoma-carcinoma sequence: a reappraisal with immunohistochemical expression of ferritin.

To examine the biological significance of ferritin (FRN) expression, a retrospective immunohistochemical study was performed in normal colonic mucosae (n = 8), adenomas (n = 88), and colorectal carcinomas (n = 104). FRN was present in some epithelia in the crypt base of normal colonic mucosae. Significant cytoplasmic staining for FRN was revealed in 26 (29.5%) cases of adenoma and 54 cases (51.9%) of adenocarcinoma. The cancer cells had a higher proportion of FRN expression than those of adenomas or non-neoplastic mucosae (P < 0.001). Expression of FRN showed a positive association with the degree of dysplasia (P = 0.039) and the distal location of adenoma (P = 0.013). FRN expression tended to be associated with the tumor size (P = 0.083), but no substantial difference was observed among the histologic types of adenoma (P = 0.754). The results suggest that cytoplasmic FRN expression is associated with cellular proliferation. The proliferative index shows a significant difference through the adenoma-carcinoma sequence. Further investigation is necessary to clarify the clinical implication of FRN expression in tumor cells and normal-appearing mucosae.

Adenocarcinoma↗

PubMed1995

Power spectral analysis of electroencephalographic desynchronization induced by cocaine in rats: correlation with evaluation of noradrenergic neurotransmission at the medial prefrontal cortex.

We applied continuous, on-line and real-time spectral analysis of electroencephalographic (EEG) signals and microdialysis to evaluate the possible participation of noradrenergic neurotransmission at the medial prefrontal cortex (mPFC) in EEG desynchronization induced by cocaine. Male Sprague-Dawley rats that were under chloral hydrate anesthesia were used. Intravenous administration of cocaine (1.5 or 3.0 mg/kg) dose-dependently induced EEG desynchronization, as represented by a decrease in root mean square (RMS) and an increase in mean power frequency (MPF) value of the EEG signals. Power spectral analysis further revealed that whereas both doses of cocaine promoted a reduction in the alpha (8-13 Hz), theta (4-8 Hz), and delta (1-4 Hz) components, the lower dose of cocaine decreased, and the higher dose increased the beta band (13-32 Hz). Microdialysis data indicated an elevation in extracellular concentration of norepinephrine at the mPFC that paralleled temporally and correlated positively with the maximal effect of cocaine on EEG activity. Bilateral microinjection of the selective noradrenergic neurotoxin, DSP4 (50 micrograms), or equimolar concentration (500 pmol) of the alpha 1-adrenoceptor antagonist, prazosin, or alpha 2-adrenoceptor antagonist, yohimbine, into the mPFC significantly blunted the decrease in delta component (prazosin) or both delta and theta components (DSP4 or yohimbine) of EEG activity by the lower dose of cocaine. On the other hand, the same pretreatments appreciably antagonized the increase in beta band by cocaine at 3.0 mg/kg. The potency of the antagonism by yohimbine, however, was higher than prazosin. These results suggest that cocaine may elicit EEG desynchronization via noradrenergic neurotransmission, and that alpha 2-adrenoceptors, and to a lesser extent, alpha 1-adrenoceptors, at the mPFC may be involved in the subtle dose-dependent changes in individual EEG spectral components.

Adrenergic Agents↗

PubMed1995

Participation of presynaptic noradrenergic fibers in the suppression of alpha 2-adrenoceptor activity by substance P at the nucleus reticularis gigantocellularis of the rat.

We applied reverse microdialysis and high performance liquid chromatography (HPLC) analysis to evaluate the participation of presynaptic noradrenergic neurotransmission in the suppression by substance P (SP) of alpha 2-adrenoceptor activity at the nucleus reticularis gigantocellularis (NRGC) in Sprague-Dawley rats anesthetized with pentobarbital sodium. Microinfusion of SP (600 microM/min) into the NRGC through a stereotaxically positioned microdialysis probe attenuated the hypotensive and bradycardiac actions of the alpha 2-adrenoceptor agonist, guanabenz (100 micrograms/kg, i.v.). This inhibitory effect correlated positively with the time course of elevation in the estimated extracellular concentrations of SP and norepinephrine (NE) in the NRGC. Direct microinfusion of NE (50 nM/min) into the NRGC also lessened the cardiosuppressant effects of guanabenz. These circulatory and NE responses to SP were, however, significantly blunted in rats in which the noradrenergic innervation in the NRGC was depleted with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) pretreatment. Microinfusion of NE into the NRGC of DSP4-pretreated animals restored the attenuation of guanabenz-induced cardiovascular suppression. These results suggest that SP may depress the activity of alpha 2-adrenoceptor at the NRGC that are involved in circulatory regulation by increasing the extracellular concentration of NE via a presynaptic modulation of noradrenergic neurotransmission.

Adrenergic alpha-Antagonists↗

PubMed1995

Influence of gender and age at onset on the HLA associations in Chinese with insulin-dependent diabetes mellitus.

IDDM in Singaporean Chinese was associated with HLA B58, DRB1*0301, DQB1*0201, and joint occurrences of DRB1*0301/*0901 and DRB1*0301/*04. Of the DR4s the frequencies of DRB1*0401, *0404, and *0405 were higher and *0406 was lower in patients compared to controls. DRB1*0301/*0901 was observed mainly in female patients and the frequency showed an inverse relationship with age at onset, whereas DRB1*0301/*04 was observed mainly in male patients and also showed an inverse relationship with age at onset. DRB1*1202 showed an increasing frequency with age at onset. IDDM patients had a higher frequency of homozygous NAsp57 DQ beta chains and a lower frequency of homozygous Asp57 DQ beta chains compared to controls, especially in younger onset patients.

Adolescent↗

PubMed1995

Excitatory innervation of caudal hypoglossal nucleus from nucleus reticularis gigantocellularis in the rat.

We examined the possible innervation of the caudal hypoglossal nucleus by the nucleus reticularis gigantocellularis of the medulla oblongata, based on single-neuron recording and retrograde tracing experiments in Sprague-Dawley rats. Under pentobarbital sodium (50 mg/kg, i.p.) anesthesia, electrical stimulation of the caudal portion of the nucleus reticularis gigantocellularis with repetitive 0.5-ms rectangular pulses increased (46 of 51 neurons) the basal discharge frequency of spontaneously active cells, or evoked spike activity in silent, hypoglossal neurons located at the level of the obex. This excitatory effect was related to the intensity (25-100 microA) and/or frequency (0.5-20 Hz) of the stimulating pulses to the nucleus reticularis gigantocellularis. Perikaryal activation of neurons by microinjection of L-glutamate (0.5 nmol, 25 nl) into the caudal portion of the nucleus reticularis gigantocellularis similarly produced an excitatory action on eight of 14 hypoglossal neurons. Retrogradely labeled neurons were found bilaterally within the confines of the nucleus reticularis gigantocellularis following unilateral microinjection of wheatgerm agglutinin-conjugated horseradish peroxidase or Fast Blue into the corresponding hypoglossal recording sites. Furthermore, the distribution of labeled neurons in the nucleus reticularis gigantocellularis substantially overlapped with the loci of electrical or chemical stimulation. These complementary electrophysiological and neuroanatomical results support the conclusion that an excitatory link exists between the nucleus reticularis gigantocellularis and at least the caudal portion of the hypoglossal nucleus in the rat.

Amidines↗

PubMed1995

T-cell receptor gene expression in tumour-infiltrating lymphocytes and peripheral blood lymphocytes of patients with nasopharyngeal carcinoma.

The T-cell receptor (TCR) repertoire expression of tumour-infiltrating lymphocytes (TILs) from 19 nasopharyngeal carcinoma (NPC) biopsies was compared with those of lymphocytes from 18 control nasopharyngeal biopsies. mRNA was extracted from these lymphocytes and the cDNA transcribed. A panel of 18 V alpha- and 21 V beta-specific primers was used to detect the TCR gene use from cDNA. The use of V alpha and V beta genes was restricted in TILs compared with lymphocytes from biopsies. The frequencies of V alpha 2, V alpha 3, V alpha 9, V alpha 10, V alpha 11, V alpha 13, V alpha 14, V alpha 15, V beta 11, V beta 15 and V beta 20 were decreased and the frequencies of V alpha 10 [Pc = 0.04; relative risk (RR) = 0.05], V alpha 11 (Pc = 0.02; RR = 0.07), V alpha 13 (Pc = 0.002; RR = 0), V alpha 14 (Pc = 0.04; RR = 0.05), V beta 14 (Pc = 0.001; RR = 0.03) and V beta 20 (Pc = 0.001; RR = 0.03) remained significantly reduced after correction for the number of families typed. The frequency of V alpha 17 was higher in NPC biopsies than in NPC PBLs (P = 0.05), and the frequency of V beta 15 was lower in NPC biopsies than in NPC PBLs (P = 0.02). The frequencies of V alpha 17 and V alpha 18 in HLA-B46+ patients were significantly lower (P = 0.009; P = 0.044) than in B46+ controls. The results suggest that the restriction of TCR gene use in NPC patients may be important in NPC pathogenesis.

Base Sequence↗

PubMed1995

Participation of noradrenergic neurotransmission in the enhancement of baroreceptor reflex response by substance P at the nucleus tractus solitarii of the rat: a reverse microdialysis study.

We applied reverse microdialysis and HPLC analysis to evaluate the participation of noradrenergic neurotransmission in modulation of the baroreceptor reflex response by substance P at the nucleus tractus solitarii in Sprague-Dawley rats anesthetized with pentobarbital sodium (50 mg/kg, i.p., with 20 mg/kg/h.i.v. supplement). Continuous infusion of substance P (600 microM) at 1 microliter/min into the nucleus tractus solitarii through a stereotaxically positioned microdialysis probe (active exchange length, 180-200 microns; diameter, 220 microns) for 1 h elicited an enhancement of the baroreceptor reflex response. This facilitatory effect correlated positively, during the 60-min infusion period, with the time course of increase in the extracellular concentration of substance P and noradrenaline in the nucleus tractus solitarii. Experimentally elevating the concentration of noradrenaline at this medullary nucleus also augmented the baroreceptor reflex sensitivity. On the other hand, depletion of the noradrenergic fibers and nerve terminals at the nucleus tractus solitarii with DSP4 diminished the enhancement of baroreceptor reflex response and the corresponding elevation in extracellular concentration of noradrenaline by substance P. Microinfusion of noradrenaline into the nucleus tractus solitarii in DSP4-treated animals, however, potentiated the baroreceptor reflex response. These results suggested that the enhancement of baroreceptor reflex response by substance P may involve an increase in the concentration of noradrenaline at the nucleus tractus solitarii via a presynaptic mechanism.

Animals↗